To isolate and characterize non-plasma (and presumably lung specific) proteins of the human lung surfactant system evaluate the distribution of these proteins in relation to the lipid composition of (a) the intracellular and alveolar surfactant of lungs obtained at autopsy from human fetuses and newborn infants with or without hyaline membrane disease (fractions of lung washings as well as lung specimens from 16 human fetuses and newborn infants stored at -80 degrees C are currently available for evaluation). (b) From amniotic fluid samples obtained at different stages of gestation. To evaluate lecithin/sphingomyelin (L/S) ratio, lipid and protein profiles and surface activity of tracheal aspirates obtained daily from intubated infants with respiratory stress. This project has already been initiated in collaboration with the Department of Pediatrics of our hospital. These aspirates are routinely taken hourly (and currently discarded) from intubated infants, and they were found to contain considerable amount of surface active material for evaluation. The above biochemical studies will be correlated with the clinical course of the patients.